1. Field of the Invention
The present invention relates to a solid-state imaging device, and more particularly to an electron multiplication type of solid-state imaging device for multiplying electrons to make signal sensitivity higher. The present invention also relates to a drive method therefor, and more particularly to a method of multiplying signal charges to read out high-quality signals.
2. Description of the Background Art
Electron multiplication in solid-state imaging devices has been proposed by, for example, Japanese patent laid-open publication No. 340099/1996, which discloses that impact ionization, which is an electron multiplication phenomenon, occurs at the read-out gate of a charge-coupled device (CCD). If impact ionization amplifies dark current, the dark current thus amplified will develop as a so-called white mark on an image. The '099 publication is aimed at preventing impact ionization from occurring. In order to suppress the occurrence of such white marks, the solid-state imaging device adjusts an impurity concentration in a semiconductor established between the charge storage and the read-out gate so that the strength of an electric field occurring on the read-out gate is alleviated, and consequently, electron multiplication is suppressed.
Another Japanese patent laid-open publication, No. 2002-290836, discloses an application of electron multiplication to a frame transfer type of CCD. The JP publication '836 has an amplifier for causing impact ionization to occur. Thus, the solid-state imaging device of the '836 publication lets electron multiplication to occur to improve sensitivity.
In solid-state imaging devices, if only signal charges could be multiplied, the signal-to-noise (S/N) ratio could be improved. However, in the case of aforementioned '099 publication, as clear from its purpose described above, the solid-state imaging device suppresses the occurrence of impact ionization per se. From the viewpoint of improving a signal-to-noise ratio, this suppression is negative use of electron multiplication.
In the aforementioned '836 publication, signal charges are multiplied after being transferred vertically, so that noise caused during this transfer is amplified at the same time. As a result, from the viewpoint of a signal-to-noise ratio, the technique disclosed by the '836 publication is not the best solution.